A costermonger sold fruit and vegetables from a barrow — a wooden cart pushed through the street — calling out prices to people passing by. The word is thoroughly historical and thoroughly London: costermongers were a famous part of Victorian street life, with their own slang, their own dress, and a reputation for being loud, quick-witted, and hard to cheat. You meet the word in Dickens, in history books, in stories about old London, and almost nowhere else. Nobody today walks into a shop and calls the owner a costermonger. It is often shortened to 'coster' in older writing. If you need the everyday modern words, they are 'greengrocer' (someone with a shop selling fruit and vegetables) or 'market trader' / 'street vendor' (someone selling from a stall). Learn 'costermonger' so you can read it, not so you can say it.